This invention relates to the complete construction and design of a high or low speed MAGLEV (Magnetic Levitated Vehicle) system. Of central concern is the means by which propulsion, levitation and stabilization forces are provided to the vehicle.
Unlike the design of many conventional electric motors, a MAGLEV system must be sensitive to the fact that one side of the motor, either the rotor or the stator, is nearly infinite in length. Thus, the components, and the cost of the track in particular, is a much greater concern than is usually the case with a conventional motor, in which both the rotor and stator are comparatively small and of finite length.
Most high speed MAGLEV vehicles are projected to run at speeds of about 150-300 mph. Not only is the aerodynamic drag a key factor in the design of the vehicle, but magnetic drag is as well. The term magnetic drag refers to the forces exerted on the vehicle by eddy currents induced in the track acting against vehicle-based magnets. At low speeds in the neighborhood of 30 mph, this drag can constitute up to 200 to 300 per cent of the total drag, while at 300 mph it might constitute 5 per cent of the total drag. Magnetic drag should be minimized, whenever possible.
Of equal importance to the design of a MAGLEV vehicle is the issue of how levitation and stabilization will be achieved. By way of comparison, all commercial aircraft land at speeds less than 170 mph since the landing wheels can not withstand impact at higher speeds. Whatever mechanism is used to achieve the levitation and stabilization of a MAGLEV vehicle, it must be guaranteed to be fail-safe when a local power substation goes down. Similarly, it cannot depend on one or two superconducting magnets which have the possibility of quenching at any time. Lastly, there must a mechanism for transferring both propulsion power and service power (lighting, heating, air-conditioning, etc.) to the vehicle.
In summary, the targeted objectives for an efficient and viable MAGLEV system are as follows:
1. A low cost track. This is the element of the system which will constitute the greatest component of its costs.
2. Means for reducing unwanted eddy currents with their commensurate heating loss and magnetic drag.
3. Means for realizing efficient levitation and stabilization in a fail-safe mode. This means should be able to realize such levitation and stabilization at both high and low speedsxe2x80x94preferably without incurring undue additional cost to the system.
4. Means for delivering propulsion and service power to the vehicle.
The background work in this area is quite extensive. Among the earliest patents proposed for a MAGLEV system is that by Maurice F. Jones and Lee A. Kilgore, U.S. Pat. No. 2,412,512 (1946). Their proposed system consisted of a laminated polyphased wound core in the vehicle which acted against a squirrel cage current rail extending the length of the track. This patent shares some features with that of Millard Smith and Marion Roberts, U.S. Pat. No. 3,233,559 (1966), which also proposed a linear electric induction motor as a means of propelling a vehicle down the track. Both suggestions suffer the problem of getting a large amount of power into the vehicle; this task might be possible at present using state of the art brushes. With the proposed squirrel cage arrangement, operation would not be possible without considerable magnetic drag loss. Frank Godsey and Maraca Jones, U.S. Pat. No. 2,666,879 (1954), suggest a similar configuration, but mount the windings along the length of the track; the vehicle need only carry a conducting sheet which would be sandwiched between these windings. The cost of such a MAGLEV configuration might be astronomical.
More recently, additional systems have been suggested which attempt to realize propulsion using linear synchronous motors. More notable among this group is the patent by Naoki Maki, et al, U.S. Pat. No. 3,913,493 (October 1975). Their system uses a linear synchronous motor in which track-based three phase coils interact with a vehicle-based magnetic coil group to realize the propulsion. Again, the cost of such a system could be extremely high.
Among the first groups to suggest an integrated system yielding levitational guidance and propulsion was that by Richard Thornton, U.S. Pat. No. 3,850,109 (November 1974). The vehicle was constructed with a number of long, thin superconducting coils which, when energized, would interact with xe2x80x9cIxe2x80x9d strips along the guideway to effect levitation. Propulsion was accomplished by interaction with other vehicle coils reacting with an armature based winding in the track. Although such a system would indeed realize levitation at very low speeds, along with propulsion, the cost could be very large. Moreover, if either of the two lift coils suffered a superconducting quench, catastrophic results would no doubt ensue.
In what might be summed up as possibly the most expensive MAGLEV system ever proposed, Ushio Kawabe and Hiroshi Kimura (U.S. Pat. No. 3,662,689 (May 1972)) suggest the use of a hard superconductor which is laid out along the length of the track and flooded with superconducting fluid. The superconductor would react against a vehicle-based magnet to induce eddy currents realizing both the levitation and stabilization. The superconductor was arranged in a box-like configuration underneath the vehicle. Also laid along the length of the track was a ladder structure. Current would be impressed from one side of the ladder, laid horizontally along the ground, to the other side of the ladder. A propulsion force would then be generated as a consequence of the interaction of the vehicle-based magnetic field with the current in this ladder using conventional Lorentz forces, i.e. {right arrow over (J)}xc3x97{right arrow over (B)}. If one examines FIG. 5 of that patent more closely, it will become apparent that the superconducting magnet in the track underneath the solenoidal magnet is used to generate the propulsion/lift field in the vehicle. The interaction of the two fields will actually cause the magnetic field to be horizontally directed in the plane of the ladder. The {right arrow over (J)}xc3x97{right arrow over (B)} forces realized would be very small indeed. This parenthetical note is important because the propulsion forces realized by the present system are not unlike those which Kawabe was attempting to realize, but had failed to. In any event, the cost of his levitation system seems hopelessly unrealistic.
Kazumi Matsui, et al., in U.S. Pat. No. 3,771,033 (November 1973), and U.S. Pat. No. 3,904,941, (September 1975), outline a means for generating propulsion forces for a MAGLEV system using current conductors in a picket fence type arrangement in the presence of a homogenous field. These patents deserve mention because the propulsion system used in the present invention also incorporates currents passing through a picket fence arrangement of conductors lying within a homogenous field. The arrangement proposed however by Matsui is quite inefficient. Over the span of a single permanent magnet field, multiply directed currents are injected into these conductors in both directions to yield {right arrow over (J)}xc3x97{right arrow over (B)} forces which are counterproductive. The proposed system overcomes this drawback and maintains a higher efficiency by allowing currents of like direction only to contribute over a common pole face of a vehicle-based magnet.
The last patent that will be mentioned in reference to mechanisms for generating propulsion forces is that of Osamu Shibuka, et al, U.S. Pat. No. 4,641,065 (February 1987). Their track consists of a rail of north-south magnets directing their flux in a predominately horizontal direction with the ground. A conducting rail of U-shaped cross-section fits around the magnets and is provided with a set of brushes for changing the direction of the current within a moving U-shaped coil. The brushes pick up current from the stationary feed line; the current interacts with the magnet field to generate the propulsion forces.
That system of propulsion is similar to one of the two embodiments used in the present invention for generating propulsion forces. The differences are quite important, however. Unlike Shibuka, the conductors in the present system are always fixed; the magnets are mounted on the vehicle to reduce the cost of the track. Secondly, the coil used in the present invention serves a multiplicity of roles and is itself a collection of four subcoils. They are never configured with a U-shaped cross section as is explicitly required in Shibuka""s patent. These alterations and differences allow for a sizeable cost reduction of the system.
In addition to achieving the propulsion forces necessary for the vehicle, it is necessary to guarantee stabilized levitation. Almost all systems require a separate assembly of magnets or superconducting coils to achieve the levitation. For example, the 20 year old German Transrapid system has been using an attractive magnetic floater type arrangement similar to the one proposed by Sakae Yamamura, et al, U.S. Pat. No. 4,646,651 (March 1987). Generally, a piece of steel is run under the guideway of the track and an active electromagnet interacts with that piece of steel to maintain the gap at a specified setting. It is necessary in these arrangements for the vehicle to wrap around the track support structure to achieve an attractive support from below. The aerodynamic losses associated with wrapping the structure, however, are large. In addition, the track support structure must have a T-shaped cross sectionxe2x80x94yet, additional cost penalties are incurred with such a design. By contrast, the suspension and guidance system used in the present invention induces the necessary levitation and stabilization forces.
Among the first to propose the use of null flux coils in electromagnetic inductive suspension systems was J. R. Powell, U.S. Pat. No. 3,470,828 (October 1969). In actuality his proposed scheme used null flux coils to realize lateral side-to-side stabilization and isolated vertically oriented coils for the levitation forces. As the vehicle-based magnets are moved vertically with respect to these outboard coils affixed to the track, additional flux is induced in the coil to generate vertical restoring forces. A rather significant problem facing all MAGLEV designers is the issue of high speed magnetic drag. The currents induced in the vertical leviation coils do indeed yield the currents necessary to produce lift on the vehicle. Commensurate with this lift, however, is a sizable drag on the vehicle. By properly choosing the size of the null flux coils used in the Powell patent for stabilization, a large percentage of that drag can be eliminated. Well designed null flux coils have been shown to yield up to a 200 to 1 lift to drag ratio.
There are two patents by Shunsuke Fujiwara, U.S. Pat. No. 4,779,538 (October 1988) and U.S. Pat. No. 4,913,059 (April 1990) which utilize null flux coils for both levitation and stabilization. In both patents, the vehicle runs down a long boxlike trough. Null flux coils are arranged along the vertical side walls of the trough to provide the levitation forces needed by the vehicle. The superconducting magnet is mounted on the vehicle and is in an orientation which drives flux horizontally through the vertical mounted null flux coils. Any displacement from the baseline equilibrium position induces currents in the coils, with commensurate forces which act to realign the magnets with respect to the null flux coils. An important part of those systems was the use of connections from the left wall of the track to the right wall of the track. By connecting the two sets of null flux coils on either side of the vehicle, it is possible to realize higher lateral side-to-side stabilization forces. Their final arrangement consisted of two sets of null flux coils, each set being arranged on either side of the vertical walls of the vehicle. The two sets are themselves joined together by connections across the track to realize a higher efficiency in side to side restorative stabilization. The aerodynamic losses associated with the vehicle running down such a contorted track, however, would be significant. Moreover, the cross connections of these null flux coils constitute a considerable additional cost in construction, since the coils must pass within or below the structure of the guideway.
By way of completeness it should be mentioned that inductive systems which do not use null flux coils have also been recommended. One such system is that of Jurgen Miericke, U.S. Pat. No. 3,834,317 (September 1974). Induction is the only principle used in this patent to realize both the vertical and horizontal restoring forces. Track-based coils induce currents in conducting plates which are arranged both vertically and horizontally down the length of the track. The drawback of that design is the presence of larger drag forces accompanying the lift.
Because the present invention employs a flux elimination principle similar to that of governing null flux coils, it is appropriate to refer to alternative systems that involve the null flux coil.
The following represents an extension of work which was presented at the IEEE Magnetics Conference-INTERMAG 95 (Apr. 18-25, 1995 San Antonio, Tex., p.AT-09). FIG. 1 shows a vehicle 1 riding on two vertical rails 2 which are supported by a guideway structure 3 which is typically made of concrete based bulb xe2x80x9cTxe2x80x9d""s. The vertical track consists of castellated conducting members 8 and null flux coils 9. The vehicle has two types of magnets in it. The first is a transverse magnetic source 11 which drives flux across the vertical rail. The second set type are repulsive magnets 12 which have no flux being driven through the rail at its center plane.
A closer view of the rail structure 2 is shown in FIG. 2. The current injected into the castellated members 8 provides the propulsion current for the vehicle. It is returned at cross-over points 10 to the adjacent rail. As the transverse magnets 11 translate past the null flux coils 9, currents are induced in the null flux coil as depicted in FIG. 3. These currents interact with vehicle-based transverse magnetic sources to yield high lift forces and small drag forces.
An alternative to using a castellated track is the series of xe2x80x9cTxe2x80x9d conductors shown in FIG. 4. Here the propulsion currents are driven into a brush contact surface 17. The currents are forced down these xe2x80x9cTxe2x80x9d shaped conductors each of which is insulated from its neighbor by an insulated spacer 18. The use of brushes and a brush contact surface greatly reduces the power handling problems and cost along the MAGLEV track.
FIGS. 1-4 have been detailed to illustrate major problems commensurate with the use of null flux coil topology. Among the disadvantages associated with that system are the following:
1. Two types of magnetic field sources are necessary on the vehicle. The repulsive magnetic field source 12 serves only to generate guidance or lateral stabilization for the vehicle, but constitutes an additional weight burden, as far as lift and propulsion is concerned.
2. The system requires two types of conductors or coils in the track. The castellated conductors 8 or the xe2x80x9cTxe2x80x9d conductor 17 serves the role of providing propulsion for the vehicle. The xe2x80x9cfigure eightxe2x80x9d coils serve the role of providing currents which are passively induced to yield levitation and guidance. From a construction and cost perspective, it would be more advantageous if a single coil could serve all three roles.
3. The flux path associated with the use of transverse magnets 11 is not conducive to a light weight vehicle. The pole face area required for sufficient operation of such a system is quite large, being at a minimum 2-3 ft2. A properly designed magnetic flux path requires a relatively large cross-sectional area of back iron to complete its path over to the next pole face. This type of magnetic field source is described as a dipole. The field falls off as the reciprocal distance cubed away from the dipole. Thus, it constitutes a secondary disadvantage in that shielding in the passenger compartment becomes somewhat problematic. It would be more beneficial if the magnetic fields were a quadrapole or octapole type field which falls off much more rapidly away from the source.
4. The castellated track approach requires the power to be injected into the rail from the wayside. The current must be injected with the proper phase with respect to the vehicle position; therefore, such an approach requires the use of transducers and complex power handling. The use of brushes and contact surfaces eliminates this complication and reduces the complexity of the system. Thus, the variant brush surface 17 which allows for current injection via brushes (as depicted in FIG. 4) offers a considerable advantage which should be retained if at all possible in the present invention.
These and other problems and disadvantages associated with the prior art are to a large extent overcome by the invention disclosed herein. The driving motivations behind the invention are the items listed in the previous section, those being to minimize the cost of the track, to realize levitation and stabilization in a fail-safe, cost-effective manner, to eliminate eddy current losses which produce additional magnetic drag, and to realize a means for delivering the power for propulsion and secondary service to the vehicle.
The basic embodiment of this invention is shown in FIG. 5a. It utilizes a vehicle 101 which moves over a track having two vertical rails 102. The track is supported by a guideway structure 103. Extending from the vehicle 101 are magnetic sources 104 which are configured to flank each of the vertical rails 102. The rais 102 house coils 105. Thus, as depicted in FIGS. 5a-c, as the vehicle 10 travels along the track, magnetic sources 104 extend downward from the vehicle 101, each source flanking one of the vertical rails 102 and, of course, flanking the composite coil 105 housed within it.
The first departure from the conventional prior art is to employ vertically stacked magnets as the magnetic source in this design, with dissimilar magnetic pole faces one on top of the other. Using vertically stacked magnets accomplishes two objectives. First, it eliminates the return flux from transverse magnet 11 which formerly travelled axially down the track in FIG. 1. Second, it contains the flux more effectively over the height of the vertical rail, and thus eases the difficulty of shielding.
With this configuration of magnets, the use of null flux coils will no longer be suitable to passively generate levitation forces for the vehicle. A new type of coil must be employed for this purpose, a coil which is, in fact, a composite collection of subcoils and thus will be referred to as the composite coil. This composite coil consists of four smaller coils which are connected in parallel as shown in FIGS. 8a and 9a. Propulsion is realized in a manner likened to that depicted in FIG. 4 by brushes which inject current into the parallel connection points of the composite coil. The closed electrical circuit created by the parallel connection of the subcoils will have currents induced in it as magnets are translated past it. As will be apparent to one of ordinary skill in the art, proper placement of these coils with respect to the magnetic field source generates both levitation and guidance forces passively. Lower resistance coils with higher L/R ratios yield greater lift at lower speeds.
The stacked magnets have a sizable leakage flux from the upper magnet to the lower. As the composite coil is laterally displaced so that it becomes closer to the magnetic field sources on the left side of the vehicle with respect to the right, currents will be induced in the composite coil due to this leakage flux which acts to generate forces which re-center the composite coil.
Because the guidance and lift forces are realized by eddy currents which are induced by the translation of magnets or a magnetic source past the coil, no lift or guidance is realized at low speeds (i.e., less than 30 MPH). Thus, an alternative means for generating both levitation and guidance must be sought. The simplest and most cost-efficient solution is to simply use wheels affixed to the vehicle. These wheels conceptionally mount within the outer frame of the vehicle and project only slightly past the outer skin. Alternatively, they can be retracted as with conventional aircraft vehicles. This type of eddy current induced lift is similar to that obtained with conventional null flux coils; both return a relatively high Lift/Drag ratio, except at very low speeds. There is a speed at which the drag peak is large (approximately 20 MPH). It is advisable to operate the vehicle at low speeds at the so called null flux point, the height at which the magnetic field sources induce no currents in the composite coil. The guideway can be constructed so that at speeds in excess of the maximum drag speed point, the guideway structure would be lowered with respect to the midline of the composite coils allowing the vehicle to induce currents and provide lift for itself. Horizontally directed wheels braced against the guideway substructure or the rail are necessary to provide for lateral guidance at low speeds. They too can be withdrawn as the vehicle speed increases and becomes adequate to provide for its own stabilization. A more costly alternative to using wheels to realize levitation at low speeds is to insert an additional coil in the vehicle into which current is injected via the brushes to provide additional lift as currents interact with the same magnets in the vehicle.
Among the advantages realized by this system are the following:
1. One composite coil effects the three functions of propulsion, levitation, and guidance.
2. The stacked magnet design lends itself to an efficient low reluctance path and eases requirements placed on magnetic shielding of the passenger compartment.
3. No cross connections of the composite coils from rail to rail are required, in contrast to Fujiwara U.S. Pat. No. 4,913,059 (April 1990).
4. The system lends itself to the use of brushes and contact surfaces, which eliminates the need for power circuitry handling along the track and complex position sensoring equipment.
An assessment of how well the targeted objectives outlined in the design requirements of this invention have been realized serve as a fitting conclusion to this summary.
The first objective was that of a low cost track. By using passive coils impregnated in a recycled plastic matrix in the track, the cost is indeed minimal because the track is passive, and the requirement of periodic power handling along the length of the track is eliminated.
The second objective was to reduce unwanted eddy currents, and the heating and magnetic drag which they cause. From field theory arguments, it is possible to show that if copper wire is used to wind the coils, each strand of which is no larger than 0.13xe2x80x3 in diameter, the eddy current heating loss can be kept to less than 10% of the I2R losses commensurate with propulsion.
The third requirement was an efficient means for generating lift and stabilization. The null flux like composite coils deliver levitation and guidance passively by induction, and can theoretically deliver high lift to drag ratios. Because this lift and guidance is being generated by a coil that is also used for propulsion, an overlap with the objective of having a low cost track is achieved.
The final requirement involved power delivery to the vehicle and to the track. By using a brush current injection, the need for position sensors along the track is eliminated as well as the required complex multi-frequency power handling along the track. Getting the power into the vehicle is usually achieved using a third rail pickup system or alternatively, with onboard generation equipment.